This invention relates to a method and apparatus for coating particulate granules, and more particularly discloses a method and apparatus for coating particulate granular products such as pills, pellets, beads, tablets, dragees, nonpareils and the like, with pharmaceutical products and chemical treatments. The method and apparatus are also appropriate for coating foods, candies, nuts, fruits and the like with such things as chocolate and sugar.
A wide variety of pelletized or granulated products have been treated in the prior art by coating those products with an additional material. Common among such products are the full range of pharmaceuticals such as medicinally coated pills, tablets and non-pareils; chemical products such as detergents; and foodstuffs such as sugar or chocolate coated candies. Coatings for such products are applied either to provide a seal, or to add an additional material to the granular base. In many instances, it is desirable that the coating be uniformily and precisely applied, and with as few surface imperfections as possible. In the case of pharmaceutical coatings, uniformity is desirable to prevent waste of valuable drugs or medicines, and to ensure both accurate measurement of dosage and accurate delivery of dosage to patients; for all coated products, uniformity and precision reduces cost, and produces a more aesthetically pleasing product.
The coating materials to be applied to the granular products are usually either powders such as sugar, or are liquids, often in an aqueous or solvent solution. Both liquid and powdered coating materials lend themselves to coating granular products by dispensing the materials onto the granular particles. Dispensing is usually accomplished by spraying, poring, or ladeling the coating material onto the granules. To ensure uniformity, and to prevent the coated granules from adhering to each other during the drying process, coating materials are normally dispensed while the granules are tumbling in a pan or drum; the tumbling action tends to spread the coating material throughout the granules. Frequently, however, mere tumbling is insufficient to provide the requisite uniformity and to prevent adhesion between moistened granules; additionally, mere tumbling often produces dimpled and aesthetically displeasing final products.
Accordingly, it is desirable to apply the coating material to the tumbling granules simultaneously with the introduction to granules of a current of a drying gas. Although the current may be of any gas, the most commonly used gas is air. The air current, which is usually heated, substantially reduces the time required to dry coating materials, and reduces damage to the granules and their coatings by allowing coating with a less vigorous tumbling. Proper control of the air temperature, air flow rate, moisture content, vigorousness of granule tumbling, and flow rate and dispersion of the coating liquid tends to produce a final product that is uniformly coated, undamaged, and possessed of an aesthetically appealing smooth surface.
In the prior art, two principal techniques have been used to coat particulate granules. The most common technique is to provide a rotating pan or drum in which pellets or granules are allowed to tumble, while dispensing coating material onto the tumbling granules and drying the granules with a stream or flow of air. A second technique involves suspending the granules in a fluidized bed produced by a current of air in a vertical tube. The granules are held in tumbling suspension by the current of air, and are coated with a liquid injected into the fluidized bed.
An apparatus typically used in the prior art for the first such technique is that disclosed in Lachman, U.S. Pat. No. 3,141,792 for Automatic Tablet Coating Apparatus. Such devices require extensive supervision, and optionally include blades or mixing baffles within the rotating pan or drum to improve mixing of the granules during the coating operation. Devices of the kind disclosed in Lachman, though effective to provide coatings to granulated products, are not optimal for the coating operation. Waste of coating materials can result from contact of the coating material with the rotating drum, and dispersion of the drying air through the coated tablets is often less than is preferred. More effective use of drying air, accomplished through increased air dispersion, is desirable to speed the drying of individual granules. More precise control of the dispensing of coating material onto the granules is also desirable, in order to lessen the amount of coating material that will contact and adhere to the pan walls. This is an especially prevalent problem when the coating material is liquid. Thus, designers of coating devices seek methods and mchanisms that increase the volume and dispersion of air throughout the granulated particles during the coating operation, and control dispensing of the coating material to reduce the contact of the moist material with the pan walls.
Increased air dispersion and faster spread of the coating can be produced by increasing the granules' rate of tumble, either through a higher rotational velocity of the pan, or by introducing more vigorous agitation with mixing baffles or blades in the pan. However, mechanically increased agitation produces a higher velocity of granule movement, and increases the kinetic energy of each collision between agitated granules in the rotating drum. Additionally, mechanical agitation produces substantial frictional interaction between particles. Such friction affects both the integrity of the coating, and abrades the granules themselves. Collisions and friction produce product loss, surface contamination, and deterioration of the coated granules. Additionally, friction and collisions not only damage the granules, but produce as well an undesirable dust that is both a health and safety hazard.
Various techniques have been applied in the prior art to increase air dispersion and minimize the damage and deterioration resulting from agitation of granules during rotational pan coating operations. One such technique is the method of coating tablets disclosed in U.S. Pat. No. 3,573,966 issued to Hostetler for a Tablet Coating Method. The method disclosed therein includes placing a charge of tablets in a rotary drum having perforations in a portion of its exterior surface. During tumbling, drying air is drawn into the drum through the drum's exterior perforations and exhausted through the charge of tumbling tablets. By controlling the rotational speed of the drum such that the agitated tablets are maintained in a localized tumbling bed, and by exhausting drying air through the perforated surface directly at the location of the bed of tablets, increased dispersion of air and enhanced drying ability can be produced.
Alternatively, particulate granules have been coated in the prior art by spraying or dispensing coating material onto a fluidized bed of granules. This method is accomplished by placing granules in a vertical tube, and injecting a continuous flow of air in the bottom of the tube to suspend the granules against the force of gravity. Liquified coating material can then be dispensed into the suspended mass of granules. An apparatus for carrying out such a method is disclosed in U.S. Pat. No. 2,799,241 issued to Wurster for Means for Applying Coating to Tablets or the Like. Although such fluidized bed systems as Wurster increase dispersion of air throughout the tumbling granules, suspension of granules often require large air flow rates and high velocity air, thereby producing vigorous agitation of particles and increased fracture, damage, and contamination of the coated granules.
A combination of the fluidized bed and mechanically produced rotational agitation is therefore desirable. One such combination is disclosed in U.S. Pat. No. 3,357,398 issued to Gross for Method and Apparatus for Coating Tablets. Gross discloses a rotating drum with a perforated surface in which a charge of tablets is agitated by the drum's rotation. Drying air is injected directly into the charge to fluidize the tablets and place them in air suspension. The drying air is then randomly exhausted through the remaining perforated outer surface of the drum. Gross thereby provides increased air dispersion similar to that of the vertical column fluidized bed technique of Wurster, but in a rotating drum. However, additional increased air dispersion and further reduced product loss from agitation is still desirable.
Because many of the prior art coating techniques result in appreciable quantities of wasted coating material, either through contact of the coating material with the vertical column or rotating drum, or through loss of the coating material into the exhaust of the drying air, it is desirable that a method and apparatus by developed that further increases dispersion of the drying air, and further reduces agitation of the particles. Moreover, collisions and frictional interaction between granules results in loss of product into the air exhaust in the prior art coating techniques. Reduction of loss of coating material to the air exhaust is thus desirable.
Accordingly, it is an object of this invention to produce a method and apparatus for coating particulate granules such as pills, pellets, beads, tablets, dragees, non-pareils and the like.
It is another object of this invention to provide such a method and apparatus that increases dispersion of drying air during the coating operation.
It is a further object of this invention to provide such a method and apparatus that reduces granule agitation and friction, and consequently reduces fracture and destruction of the coated granules.
Yet another object of this invention is to produce such a method and apparatus that reduces the abrasion and quantity of dust produced from granule or particle collisions.
Still another object of this invention is to produce a method and apparatus for coating particulate granules that reduces loss of the coating material to the apparatus and air exhaust.
A further object of this invention is to produce a method and apparatus for coating particulate granules that improves control of the movement of air through the granules being coated.
Still another object of this invention is to produce a method and apparatus for coating particulate granules that improves mixing of the granules being coated and promotes uniform coating of the granules.
Yet another object of this invention is to provide a method and apparatus for coating particulate granules that adjustably controls dispersion of air through the granules being coated.